The goal of this proposal is to understand the interactions between measles virus and its cellular receptor in order to provide a basis for the design of anti-viral drugs. Despite an available vaccine, measles remains one of the major causes of child death worldwide, and our work will guide efforts to find new ways to control this pathogen. We will determine the three- dimensional structures of measles virus hemagglutinin and of human CD46, a measles virus receptor. The physiological role for CD46 is as a cofactor that accelerates the deactivation of the third and fourth complement components, thereby protecting host cells from complement-dependent attack. It appears that measles virus and the physiological ligands for CD46 bind to discrete parts of the molecule. Specifically, the extracellular portion of CD46 consists of four short consensus repeats (SCRs) that contain binding sites for the measles virus hemagglutinin (in SCR1-SCR2) and complement regulatory activity (in SCR2-SCR4). One can therefore envisage the discovery of selective anti-viral inhibitors. The experiments described in Specific Aim 1 concern the expression, crystallization and X-ray structure determination of several two-domain CD46 fragments as well as a fragment that comprises all four SCRs. Since the molecule probably has some interdomain flexibility, studying the two-domain fragments increases the likelihood of obtaining crystals that can diffract to molecular resolution. Crystals for the SCR1-SCR2 fragment have already been produced, and the structure determination of this fragment has been achieved. In Specific Aim 2, we will use this structural information as a guide to carry out site-directed mutagenesis experiments and measles virus binding studies that will probe the interactions with the virus in more detail. In parallel, we will express and crystallize a soluble form of the measles virus hemagglutinin protein that contains the CD46 binding site, and we will determine its crystal structure as part of Aim 2. These structural studies are a necessary antecedent for rational drug design against measles virus. They are also likely to provide insights into strategies for vaccine and drug design against paramyxoviruses such as respiratory syncytial virus in general.